The invention relates to an optical arrangement with a light source, which emits coherent light of a specific wavelength, and with an optical Littrow grating, arranged at a specific Littrow angle xcex8L, with a multiplicity of parallel diffraction structures, following one another periodically at an interval of in each case one specific grating period and arranged on a substrate predetermining a base area.
The invention further relates to a Littrow grating for use in an optical arrangement with a light source which emits coherent light of a specific wavelength, arranged at a specific Littrow angle xcex8L, with a multiplicity of parallel diffraction structures, following one another periodically at an interval of in each case one specific grating period and arranged on a substrate predetermining a base area.
The invention further relates to a use of a Littrow grating.
An optical arrangement and a Littrow grating of the kind initially mentioned are known from U.S. Pat. No. 6,067,197 A. There reference is made to the fact that the reflection efficiency of the Littrow grating is increased if it is operated in Littrow diffraction orders for which no radiation with an even higher diffraction order exists. The Littrow grating in the optical arrangement of U.S. Pat. No. 6,067,197 A, after calculation there for light of a KrF laser incident with TM polarisation at 248.4 nm in the 95th Littrow diffraction order, has a reflection efficiency of approximately 75%. Calculations by the applicant resulted in a lower reflection efficiency in the range of 68% for a comparable optical arrangement. This difference can probably be attributed to different assumptions in calculating the reflection efficiency. Losses linked to this reflection efficiency in the order of magnitude of a third of the irradiated energy represent a considerable limitation for many applications.
It is therefore the object of the present invention to develop further an optical arrangement of the initially mentioned kind in such a way that its reflection efficiency is raised.
This object is achieved according to the invention in that the light wavelength, the grating period and the Littrow angle xcex8L are tuned to one another in such a way that the grating is used in one of the largest diffraction orders m for light reflected back at a Littrow angle xcex8L, which still fulfils the condition:
(2(m+1)/mxe2x88x921) sin (xcex8L)xe2x89xa71 
Hereinafter the last still propagating Littrow diffraction order is understood to be that for which no further radiation of higher diffraction order exists which would emerge at an even larger emergent angle than the Littrow angle xcex8L, in other words as even flatter related to the base area of the substrate.
According to the invention it has been recognised that the reflection efficiency of a generic optical arrangement can be raised in that not just any of the Littrow diffraction orders which is the last still propagating diffraction order is selected, but that further selection is made among the Littrow diffraction orders which are the last still propagating diffraction orders. From the cited condition an optical arrangement results with a Littrow diffraction order which is one of the largest among the last still propagating diffraction orders. The reflection efficiency for a Littrow diffraction order selected in this way is the highest, compared with all the other diffraction orders which fulfil the condition of being the last still propagating Littrow diffraction order. Targeted selection according to the above condition therefore leads to an optical arrangement with increased reflection efficiency.
The light of the light source preferably has a TM polarisation in respect of the Littrow grating. There is a relatively large increase potential for reflection efficiency for this polarisation direction, as is known, TM polarised light is usually reflected with lower efficiency than TE polarised light. However, according to the above selection, an increase in reflection can also be achieved for TE polarised light.
Preferably a light wavelength is used which is smaller than 350 nm. An optical arrangement with a light wavelength of this kind acts, e.g. as an end mirror of a laser resonator, for which the increase in reflection efficiency can be particularly well used.
A further object of the invention is to develop further a Littrow grating of the kind initially mentioned in such a way that its reflection efficiency is raised.
This object is achieved according to the invention by a Littrow grating in which the light wavelength, the grating period and the Littrow angle xcex8L are tuned to one another in such a way that the grating is used in one of the largest diffraction orders m for light reflected back at a Littrow angle xcex8L, which still fulfils the condition:
(2(m+1)/mxe2x88x921) sin (xcex8L)xe2x89xa71 
A Littrow grating of this kind can be configured in such a way that the advantages of the increase in efficiency by targeted selection of the Littrow diffraction order can be used for a series of optical arrangements, which differ slightly in the light wavelengths or the Littrow angles. Further advantages of the Littrow grating emerge from those depicted above in connection with the optical arrangement.
The diffraction structures of the Littrow grating can in each case have a blaze flank inclined substantially at the Littrow angle xcex8L to the base area. Structuring of this kind leads to even further optimised reflection efficiency.
The diffraction structures can therein comprise a counter-flank, wherein the blaze flank and the counter-flank form at the vertex of a diffraction structure an apex angle which is smaller than 90xc2x0. Alternatively the apex angle can also be larger than or equal to 90xc2x0.
The Littrow grating can consist of quartz glass. In this material the diffraction structures can be relatively easily produced with good optical quality. Alternatively the Littrow grating can also consist of some other dielectric material.
The Littrow grating can have a coating which increases reflectivity. It can in this case consist of a material easily processed in optical quality, which does not itself reflect the radiation, as is the case with Quartz glass or silicon, for example.
The coating can be an aluminium coating. A coating of this kind is relatively economical to produce and maintains a very good reflection efficiency, in particular for UV light.
A further object of the invention is to cite a use for a Littrow grating according to the invention, in which the dependency of the largest diffraction order m, for which now no output radiation, reflected at a larger emergent angle than the incident angle, of higher diffraction order exists, on the incident angle for irradiated light and therefore selectivity of the reflection efficiency in respect of the incident angle can be used.
This object is achieved by the use of a Littrow grating according to the invention to reduce the divergence of the light source. If the irradiation angle into the Littrow grating according to the invention is chosen in such a way that there are spectral portions within the irradiated light beam with irradiation angles for which an output radiation of higher diffraction order with an emergent angle larger than the irradiation angle results, these portions are reflected by the Littrow grating with less efficiency than those portions of the incident light beam for which no output radiation of this kind of higher diffraction order exists. Distribution of the light beam into the portions reflecting back efficiently and less efficiently can be influenced via the irradiation angle of the main ray of the light beam.
This further object is further achieved by using a Littrow grating according to the invention as the end mirror in a laser resonator for narrowing bandwidth.